In order to maintain signal integrity, the design of high speed integrated circuit (IC) devices requires specific characteristic impedance of conductors which interconnect an IC chip to a printed wiring board or the next level of interconnection. For example, clock circuit drivers of many high frequency circuits are designed for 50 ohm matched impedance, and this in turn necessitates a specific geometric conductor design, and a predetermined output location for the conductor system in a package or substrate to match the circuit design. Moreover, the conductor system within a substrate or package used for both single ended and differential signal transmission has different impedance levels for each mode, and thus a specific conductor design for individual devices.
Arrays of conductors provide transmission lines in semiconductor packages or substrates; these arrays or systems include signal layer(s), and a ground conductor(s) which may be within the substrate, the printed wiring board or the semiconductor chip itself. The conductive layers are interspaced with dielectric materials. Characteristic impedance of the conductors is a function of conductor spacing, dielectric thickness, conductor width, the electrical properties of the materials, and the velocity of signal propagation on the line.
Differential impedance between a pair of conductors is either odd mode or even mode depending upon the direction of signal propagation, and for some circuits the need for both modes exists within the same device. This is particularly true of higher frequency and microwave devices designed for even mode impedance, often at 100 ohms.
The ongoing proliferation of high speed circuits with specific conductor requirements places a difficult and costly demand for as many package or substrate designs. A need exists for more flexible conductor systems which accommodate a number of different circuit design requirements.